Use of digital techniques is popular in the field of electronics with the development of microprocessors, and the digital technique is used in analog equipments. A digital-to-analog (D/A) converter is employed to process a digital signal in the analog equipment. It is necessary to measure electronic characteristics of the equipment for insuring the performance thereof. One of the important electrical characteristics is a frequency domain (frequency and phase) response. A swept frequency sinewave test signal is useful for measuring the frequency domain characteristic of the analog equipments. However, since this test signal is analog, it cannot be applied to the digital equipments. The frequency response of the D/A converter may be measured by varying the clock frequency thereto, but the sin x/x correction will not hold constant with frequency variations of the clock.